1. Field of the Invention
The invention relates to a process for melting and refining vitrifiable materials for the purpose of continuously feeding glass-forming plants with molten glass.
2. Discussion of the Background
More particularly intended are plants for forming flat glass such as float or rolling plants, but also plants for forming glassware of the bottle or flask type, plants for forming glass fibres of the mineral wool type for thermal or acoustic insulation or else textile glass fibres called reinforcing fibres.
A great deal of research has been carried out on these processes, which schematically comprise a first melting step followed by a refining step intended to condition the molten glass thermally and chemically and in eliminating therefrom any batch stone, bubbles or any cause of defects appearing after forming.
In the melting range, it has thus been sought, for example, to speed up the melting process or to improve its energy efficiency. Mention may thus be made of the process consisting in rapidly heating the vitrifiable materials in a homogeneous and controlled manner while carrying out intense mechanical stirring allowing the still-solid vitrifiable materials to be brought into intimate contact with the already-liquid phase. This process is especially detailed in Patents FR-2,423,452, FR-2,281,902, FR-2,340,911 and FR-2,551,746 and generally uses electrical heating means of the submerged-electrode type.
Another type of melting process has been developed, for example of the type of those described in Patents U.S. Pat. No. 3,627,504, U.S. Pat. No. 3,260,587 or U.S. Pat. No. 4,539,034 which consist in using, as heating means, submerged burners, that is to say burners fed with gas and air, these generally being placed so as to be flush with the bottom wall so that the flame develops within the mass of vitrifiable materials during liquefaction.
In either case, although it is possible actually to very significantly reduce the residence time of the vitrifiable materials in the melting chamber and to considerably increase the production efficiency compared with xe2x80x9cconventionalxe2x80x9d melting operations, the molten glass being molten is, on the other hand, in the form of a foam which is difficult to refinexe2x80x94it is especially difficult to guarantee the quality of the final glass, especially optical glass.
Research has also been conducted in the refining field. Thus, it is, for example, known from Patent EP-775,671 and Patent U.S. Pat. No. 4,919,697 to carry out at least part of the refining operation under reduced pressure, thereby making it possible, for example, to obtain glass very low in sulphates and with a high redox. However, such refining causes intense foaming, which may be difficult to control and remove.
The object of the invention is therefore to improve melting and refining processes, aiming especially to use plants which are more compact and/or have greater operating flexibility and/or greater production efficiency, etc., without these industrial advantages being obtained to the detriment of the quality of the glass produced.
The subject of the invention is firstly a process for melting and refining vitrifiable materials, which is characterized by the combination of two characteristics:
on the one hand, all or part of the thermal energy necessary for melting the vitrifiable materials is supplied by the combustion of fossil fuel(s) with at least one oxidizer gas, the said fuels/gas or the gaseous products resulting from the combustion being injected below the level of the mass of vitrifiable materials,
on the other hand, the refining of the vitrifiable materials after melting comprises at least one step of subjecting them to subatmospheric pressure.
There has in fact proved to be an extremely advantageous synergy from an industrial standpoint between the use of melting called hereinafter xe2x80x9cmelting by submerged burnersxe2x80x9d for the sake of simplicity and that of refining at reduced pressure.
However, since this combination is far from being imposed as evidence, it might be expected that all these advantages mentioned above would be obtained only at the price of mediocre glass quality, which has not been the case. This is because, although the principle of reduced-pressure refining was known in its generality, it remained difficult to use and the user was not sure of obtaining the same acceptable residual level of bubbles/batch stones as with conventional refining. In the invention, this very particular refining is used by changing a size parameter, namely instead of feeding the refining zone with xe2x80x9cconventionalxe2x80x9d molten glass to be refined, it is fed here in fact with a glass obtained by melting by submerged burners, that is to say with glass having very special characteristics in the sense that it is foamy throughout, with a relatively low density compared with that of a standard glass. Nothing would suggest that it would be possible to refine an initially relatively foamy glass at reduced pressure.
Surprisingly, this has proved to be possible as it has been discovered that this foamy glass resulting from melting by submerged burners also had the characteristic of containing only an extremely small amount of sulphates, which may or may not have been present initially. The sulphate content is generally less than 600 and even less than 200 or less than 100 ppm, or indeed less than 50 ppm, expressed by weight of SO3 in the glass leaving the melting chamber, this being so without having to control or reduce the amount of sulphate normally contained in the batch materials used, unintentionally, or even by intentionally adding sulphates to the vitrifiable materials. It is this low amount of sulphate which allows effective refining under reduced pressure without any problem. In contrast, a high or simply xe2x80x9cstandardxe2x80x9d sulphate content in the glass to be refined would have caused, during reduced-pressure refining, a very high expansion of the foam by desulphation, which expansion would have been very difficult to control. The fact that there is almost no sulphate in the glass leaving the melting chamber may especially be explained by the partial pressure of water generated by the combustion by burners submerged in the vitrifiable materials.
It should be noted that a desulphated glass gives fewer problems of volatile compounds in the float bath, fewer risks of the formation of tin sulphide and therefore, finally, fewer risks of a tin defect in the sheet of glass.
Another highly advantageous characteristic of the glass leaving the melting chamber according to the invention should also be noted: although it is actually in the form of a kind of foam which remains to be refined, it is possible to control the size of the bubbles which it contains and, especially in certain cases, to remove almost all the smallest bubbles, that is to say those having a diameter of less than 200 xcexcm, by carrying out, on this glass while it is being melted, a kind of xe2x80x9cmicrorefiningxe2x80x9d prior to the actual refining after the melting, this microrefining facilitating the coalescence of the bubbles and the disappearance of the smaller bubbles in favour of the larger ones and being promoted by the addition into the vitrifiable materials of refining promoters of the coke or sulphate type. Furthermore, this glass leaving the melting chamber generally has a particularly low residual amount of batch stone, thereby also, just like the size of the bubbles, facilitating the refining operation after the melting operation.
The invention therefore makes it possible to have glasses which are very low in sulphate even before the refining operation, therefore glasses which are at least as low, or even deplete, in sulphate after refining, this being so without having to purify/select vitrifiable materials so that they are low in sulphate. On the contrary, it is even possible to add sulphate at the start, something which is completely surprising and advantageous.
One advantageous effect obtained by the combination according to the invention relates to the energy cost of the process: melting by submerged burners makes it possible to avoid using electrical melting of the submerged-electrode type, the cost of which may be very significant depending on the country. Furthermore, and this is the most important point, melting by submerged burners creates convective stirring within the vitrifiable materials during liquefaction, as explained in detail below. This very strong mixing between materials not yet liquefied and those which are already molten is extremely effective and makes it possible to achieve, for vitrifiable materials of the same chemical composition, melting at a lower temperature and/or melting which is much more rapid than with conventional heating means. Refining under reduced pressure also makes it possible to refine the glass at a lower temperature and much more rapidly. This is because lowering the pressure during refining causes an increase in the molar volume of the gases contained in the molten vitrifiable materials, hence an increase in the volume of bubbles that they contain and consequently an increase in their rate of rise to the surface of the bath and to their rate of removal.
By refining at reduced pressure it is possible xe2x80x9cto be allowedxe2x80x9d to work at lower temperatures than in conventional refining operations, actually within the lower temperatures used in the technique of melting by submerged burners.
The temperatures encountered both in melting and refining according to the invention are therefore completely compatible with and matched to each other, and are everywhere lower than in the usual processes, something which is economically very advantageous, simply in terms of energy cost, but also by the selection of refractory-type materials used in the manufacture of the plantsxe2x80x94materials which are less hot corrode more slowly.
The residence times in the melting and refining zones are also very significantly reduced and are compatible, this obviously having a very positive effect on the production efficiency and on the output of the plant in its entirety. At the same time, the invention makes it possible to obtain plants which are very compactxe2x80x94this is because melting by submerged burners, again due to the very strong mixing that it causes, allows the size of the melting chamber to be considerably reduced. Furthermore, refining under reduced pressure has the same consequences on the size of the compartment(s) where this operation is carried outxe2x80x94overall, the plant may therefore by very compact, with clear advantages in terms of construction cost, of operating simplification, of reduction in the wear of the structural materials, etc.
With regard to the melting operation, the oxidizer chosen may, according to the invention, be based on air, on oxygen-enriched air or even substantially based on oxygen. A high oxygen concentration in the oxidizer is in fact advantageous for various reasons: it thus reduces the volume of combustion smoke, this being favourable from the energy standpoint and avoiding any risk of excessive fluidization of the vitrifiable materials which could cause them to splash onto the superstructures or roofs of the melting chamber. Furthermore, xe2x80x9cthe flamesxe2x80x9d obtained are shorter and more emissive, allowing more rapid transfer of their energy to the vitrifiable materials and secondarily making it possible to reduce, if desired, the depth of the xe2x80x9cbathxe2x80x9d of vitrifiable materials being liquefied. We speak here of xe2x80x9cflamesxe2x80x9d, but these are not necessarily flames in the usual sense of the term. We may speak, more generally, as in the rest of the text, of xe2x80x9ccombustion regionsxe2x80x9d. Furthermore, any emission of polluting NOx gas is thus reduced to the minimum.
With regard to the selection of the fuel, this may or may not be of the gaseous fossil fuel type, such as natural gas, propane, fuel oil or any other hydrocarbon fuel. It may also be hydrogen. The process of melting by submerged burners according to the invention is therefore an advantageous means of using hydrogen, which is, moreover, difficult to use with xe2x80x9coverheadxe2x80x9d, non-submerged, burners, given the low-emissivity character of the flames obtained by H2/O2 combustion.
Combining the use, in melting by submerged burners, of an oxygen oxidizer and of a hydrogen fuel is a good means of ensuring effective heat transfer of the energy from the burners to the molten glass, leading moreover to a completely xe2x80x9ccleanxe2x80x9d process, that is to say without the emission of nitrogen oxides, NOx, or of greenhouse gases of the COx type, other than that which may arise from the decarbonization of the batch materials.
Advantageously, the melting is carried out according to the invention in at least one melting chamber which is equipped with burners which are placed so that their combustion regions or combustion gases develop in the mass of vitrifiable materials during melting. They are thus made to pass through its side walls and/or the bottom wall, and/or to suspend them from the top, fastening them to the roof or to any suitable superstructure. These burners may be such that their gas supply pipes are flush with the wall through which they pass. It may be preferable for these pipes to xe2x80x9centerxe2x80x9d, at least partly, the mass of vitrifiable materials so as to prevent the flames from being too great near the walls and not to cause premature wear of the refractory materials. It is also possible to choose to inject only the combustion gases, the combustion regions being produced outside the melting chamber proper.
As mentioned above, it has turned out that this method of heating caused intense convective stirring of the vitrifiable materialsxe2x80x94convection loops thus form on each side of the combustion regions or xe2x80x9cflamesxe2x80x9d or streams of combustion gases, permanently mixing the molten and not yet molten materials very effectively. This thus results in the highly favourable characteristics of xe2x80x9cstirredxe2x80x9d melting, without having to make use of mechanical stirring means which are not very reliable and/or subject to rapid wear.
Preferably, the height of the mass of vitrifiable materials in the melting chamber and the height at which the combustion regions or gases resulting from the combustion develop are adjusted so that these combustion regions/gases remain within the mass of the said vitrifiable materialsxe2x80x94the aim is thus to allow the convective circulation loops to be established in the material during liquefaction.
In general, this type of melting makes it possible to considerably reduce the emission of any type of dust in the melting chamber and of any gas of the NOx type since heat exchange takes place very quickly, thereby avoiding the temperature peaks likely to be conducive to the formation of these gases. It also considerably reduces the emission of gases of the COx type.
Optionally, the melting operation may be preceded by a step of preheating the vitrifiable materials to a temperature which is, however, markedly less than that necessary to liquefy them, for example to at most 900xc2x0 C. In order to carry out this preheating operation, the thermal energy of the smoke may advantageously be recovered. By thus extracting the heat from the smoke, the specific energy consumption of the plant may be decreased overall.
The vitrifiable materials may comprise batch materials, but also cullet or even scrap intended to be vitrified. They may also comprise combustible elements (organic matter): it is thus possible to recycle, for example, mineral fibres which have been sized with binder (of the type used in thermal or acoustic insulation or of thos used in the reinforcement of plastics), window panes laminated with sheets of polymer of the polyvinyl butyral type, such as windscreens, or any type of xe2x80x9ccompositexe2x80x9d material which combines glass with plastics, such as certain bottles. It is thus possible to recycle xe2x80x9cglass/metal or metal compound compositesxe2x80x9d such as window panes functionalized with coatings containing metals, these being difficult hitherto to recycle since this would run the risk of gradually enriching the melting chamber with metal which would build up on the surface of the bottom wall. However, the stirring caused by the melting according to the invention prevents this sedimentation and thus allows, for example, window panes coated with layers of enamel, with layers of metal and/or of various connection elements to be recycled.
The subject of the invention is also the recycling of all these composite elements containing glass because of the melting by submerged burners in a glass furnace. In particular, furnaces with submerged burners may be provided, the essential function of which is the manufacture of a cullet from these various materials to be recycled, which particular cullet may then serve, possibly combined with standard cullet, as batch materials for conventional glass furnaces.
Advantageously, provision may be made to introduce all or part of the vitrifiable materials into the melting chamber below the level of the mass of vitrifiable materials being melted. Some of these materials may be conventionally introduced from above the mass being liquefied and the rest from below, for example by supply means of the feed-screw type. The materials may thus be introduced directly into the mass being liquefied, at a single point or at various points distributed over the walls of the melting chamber. Such an introduction directly into the mass of materials being liquefied (hereafter referred to as the xe2x80x9cmeltxe2x80x9d) is advantageous for more than one reason: firstly, it considerably reduces any risk of batch materials flying off above the melt, and therefore reduces the amount of solid dust emitted by the furnace to the minimum. Thus, it allows better control of the minimum residence time of the said materials before they are extracted into the refining zone and allows them to be selectively introduced at the point where the convective stirring is the strongest, depending on the arrangement of the submerged burners. This or these points of introduction into the melt may thus be near the surface or more deeply in the melt, for example at a melt height of between ⅕th and ⅘ths of the total height of the melt above the level of the bottom wall, or else between ⅓ and ⅔ of the said height.
It has been seen that the process according to the invention made it possible to recycle plastics in the form of composite products combined most particularly with glass, these plastics thus serving as part of the fuel. It is also possible, and advantageous, to introduce all or part of the fuel necessary for the melting by submerged burners in the form of a solid fuel (polymer-type organic materials or coal) or even a liquid fuel, this fuel being a partial substitute for at least the liquid (especially fossil) or gaseous fuels feeding the burners. In general, the term xe2x80x9cvitrifiable materialsxe2x80x9d or xe2x80x9cbatch materialsxe2x80x9d used in the present text is intended to encompass the materials necessary for obtaining a glassy (or ceramic or glass-ceramic) matrix, but also all the additives (refining additives, etc.), all the optional liquid or solid fuels (plastic of composite or non-composite material, organic matter, coal, etc;), and any type of cullet.
The process according to the invention may operate with a high level of cullet.
As mentioned above, the refining according to the invention is therefore carried out on molten vitrifiable materials of the glass type in the foamy state. Typically, this foam has a relative density of approximately 1 to 2 for example (to be compared with a relative density of about 2.4 in the case of non-foamy glass), preferably a sulphate content of at most 100 or even of at most 50 ppm expressed as weight of SO3 and most of the bubbles having a diameter of at least 200 xcexcm. It may thus have a density of between 0.5 and 2 g/cm3, especially 1 to 2 g/cm3.
In order to improve the performance characteristics of the refining operation, various refining promoters are preferably added to the vitrifiable materials, the aim being especially to remove from the glass any bubbles having a diameter of less than 200 xcexcm right from the melting stage, as mentioned above. These may be reducing additives, such as coke (which also allows the redox of the glass to be adjusted). In this case, it is advantageous to select coke powder which has an average particle size of less than 200 xcexcm. They may also be sulphates. Refining under reduced pressure causes the bubbles to grow, the aim being for this growth to occur rapidly and that it is possible to remove and burst the bubbles on the surface of the melt easily. Other refining promoters will be effective rather more during the stage of the refining proper, after the melting stage. They allow the foam to be xe2x80x9cdestabilizedxe2x80x9d: they may, for example, be fluorine or a fluorine or chlorine compound, more generally halides, or else a nitrate of the NaNO3 type; fluorine seems to lower the viscosity of the glass and thus helps to drain the films which form between the bubbles, which draining promotes collapse of the foam. It also lowers the surface tension of the glass.
Another factor influencing the way the bubbles grow during the refining under reduced pressure is the nature of the gases above the molten material. It is possible, of course, simply to choose a partial pressure of air. It is also possible to choose to enrich the atmosphere with an inert gas of the nitrogen type, or even to choose only a partial pressure of an inert gas of the nitrogen type. This is because it has been noticed that choosing a residual pressure of an inert gas of the nitrogen type was favourable to bursting of the bubbles on the surface during the refining operation. In fact, it is too high a concentration of oxidizing gas of the O2 type which seems unfavourably to tend to reduce this bursting.
Advantageously, the subatmospheric pressure at which at least part of the refining is carried out is less than or equal to 0.5 atmospheres (0.5xc3x97105 Pa), especially about 0.3 to 0.01 atmospheres (approximately 3xc3x97104 to 0.1xc3x97103 [sic] Pa).
Advantageously, the process according to the invention makes it possible to carry out the melting and/or refining at temperatures not exceeding 1400xc2x0 C., especially at 1380 or 1350xc2x0 C.
According to a first variant, the refining according to the invention may be carried out in at least one static compartment (one which does not move during operation) downstream of the melting chamber, at least one zone in the static chamber being at a reduced pressure.
According to a second variant, the refining is always carried out downstream of the melting chamber but in a compartment capable of being rotated so as to ensure centrifugal refining, with at least one zone of the said compartment, especially the furthest upstream, at a reduced pressure.
A third variant consists in a combination of the above two, especially by using, for the refining operation, a first compartment which is static with a zone under reduced pressure and then a second compartment which rotates and also comprises a zone under reduced pressure, the pressure preferably being lower than in the static compartment.
According to one way of carrying out the process according to the invention, provision is made to treat the flux of molten vitrifiable materials, between the melting phase, by at least one flow-dividing means. This means, for example an element drilled with holes via which the flow of molten glass is forced to pass, makes it possible to divide this flow into a large number of small-diameter streams. The size of the holes is advantageously chosen so as to be close to the size of the bubbles which it is desired to remove. Thus, if the flow-dividing means is placed just downstream of the atmospheric-pressure zone of the refining compartment, the reduced pressure will act very rapidly on the streams generated by the flow-dividing means and allow rapid refining, even with very large glass throughputs. The feeding of the refining compartment with glass to be refined may thus become as it were similar to that obtained by a die emerging in a reduced-pressure chamber.
(In the context of the invention, the terms xe2x80x9cupstreamxe2x80x9d and xe2x80x9cdownstreamxe2x80x9d refer to the direction of flow of the glass through the plant from the point where the vitrifiable materials are fed into the melting chamber to the point where the refined glass is extracted).
The melting/refining process according to the invention allows glasses of highly varied compositions and properties to be manufactured. Moreover, it makes it possible, because of its low inertia, to switch from one composition to another with very short transition times.
It thus allows relatively reduced glasses, especially those having a redox of greater than or equal to 0.3, to be manufactured. (The redox is defined as the ratio of the ferrous iron FeO content, as a percentage by weight, to the total iron content by weight of the composition expressed in the form of Fe2O3).
It also allows glasses having a high SiO2 content, for example at least 72 or even at least 75% by weight, to be manufactured, these glasses generally being difficult to melt but advantageous, especially in terms of batch material cost, because they have a low density and are very compatible with plastics. It also makes it possible to manufacture quite special glasses, having a high alkaline-earth oxide content, for example containing at least 18% by weight of CaO, which glasses are, however, quite corrosive using the conventional melting processes at a higher temperature than in the invention, as well as glasses having a low sodium oxide content of at most 11% by weight for example, or having a very low sulphate content, for example of at most 100 ppm. Glasses containing iron, with a high redox but a low sulphate content also allow glasses to be obtained which have a residual blue colour which is particularly attractive and sought after in the field of flat glass for motor vehicles and for buildings, for example. Highly selective solar-protection glasses may thus be obtained on which may be deposited solar-protection layers in order to enhance the thermal performance characteristics thereof, for example layers of the TiN type, these being described especially in Patents EP-638,527 and EP-511,901.
The subject of the invention is also a melting and refining apparatus which is especially suitable for implementing the process described above and which comprises:
at least one melting chamber equipped with burners which are fed with fossil fuel(s) of the (natural) gas type and with oxidizer(s) of the air or oxygen type, the said burners being placed so as to inject these gases or the gases resulting from the combustion below the level of the mass of vitrifiable materials introduced into the said melting chamber,
at least one refining compartment downstream of the melting chamber and comprising at least one zone which can be subjected to subatmospheric pressure.
Advantageously, as mentioned previously, the melting chamber may be equipped with at least one means of introducing vitrifiable materials below the level of the melt, especially at least two of them, preferably in the form of an opening (or openings) in the associated wall(s), with a supply means of the feed-screw type. The risks of dust flying off are thus minimized, while at the same time also optionally allowing the introduction, above the melt, of the vitrifiable materials, such as silica, on which a preheating operation may be carried out without the risk of them setting solid.
Independently of the refining operation too, the invention also depends on design improvements with regard to the walls of the melting chamber which are intended to be in contact with the melt. Several variants are possible. In certain cases, known oxide-based refractory materials may be simply used, such as alumina, zirconia, chromium oxide and so-called AZS refractories. It is generally preferred to combine them with a cooling system involving the circulation of a fluid of the water type (water jacket). The water jacket may be placed on the outside, the refractories then being in direct contact with the glass, or on the inside. The water jacket then has the function of creating a cooler stream of glass near the refractories, these being particularly stressed in this context as the melt generated by the submerged burners causes strong convective currents against the walls.
Another variant consists in using, in the melt zone, not refractories but only the abovementioned water jacket;
Another variant consists in using refractory materials (optionally combinet with a cooling system of the water-jacket type) and in lining them with a lining made of a highly refractory metal such as molybdenum (or an Mo alloy). This lining may advantageously be held at some distance (for example from 1 to a few millimeters) from the walls of the refractories and may present the melt with a continuous contact surface (solid plate or plates made of Mo) or a discontinuous contact surface (Mo plate or plates drillet with holes). This lining has the purpose of mechanically preventing direct convection of the glass onto the refractories by generating a xe2x80x9cstillxe2x80x9d layer of glass along the refractories, or even by preventing any contact of the glass with the latter.
In the melting chamber, all or some of the submerged burners are preferably designed so that they can inject, into the melt, a fluid which does not participate in the combustion by substituting (temporarily) for the oxidizer and/or the fuel. This fluid may be an inert gas of the N2 type or a coolant of the liquid-water type which immediately vaporizes in the melt. The fact of thus temporarily stopping the combustion, while continuing to inject a fluid at the burner, generally has two objectives: either it is desired to stop the operation of the burner and more generally, for example, of the melting chamber in its entirety, the injection of inert gas of the N2 type allowing the chamber to be made safe in the region of the burners, or it is desired to change the burner for another while the other burners are operating and while it is therefore still in the presence of a glass melt. In this case, as explained in detail below, spraying water suitably via the burner allows the glass above the burner to be temporarily frozen, creating a kind of xe2x80x9cbellxe2x80x9d, which allows a time long enough to carry out the change without glazing the burner.
As mentioned above, the apparatus according to the invention may be provided with a flow-dividing means between the melting chamber and the refining compartment, especially just at the inlet of the refining compartment or in its furthest upstream part. This may be an element drilled with holes of suitable size.
Moreover, it should be noted that using such a flow-dividing means may also be envisaged independently of the melting means adopted: such a dividing means allows more rapid refining, with large glass throughputs, whatever the manner in which the glass is melted, for example by conventional means of the overhead (non-submerged) burner type and/or by electric melting using submerged electrodes.
Likewise, it may be advantageous to use it even if the refining operation is carried out at atmospheric pressure.
However, it is particularly advantageous to employ it in a context of melting by submerged burners which tends to generate a foam having a very high bubble content and/or in a context of refining under reduced pressure, as it considerably increases its effectiveness, which is already particularly high.
According to a first variant mentioned above, the refining compartment is static and in a vertical orientation (i.e. its height is significantly greater than its floor dimensions). This compartment comprises, according to a first embodiment, an approximately vertical internal partition which defines, in combination with the internal walls of the compartment, at least two channels. These consist, in succession, of a first channel which forces the molten vitrifiable materials to follow an ascending path and then a second channel which forces the said vitrifiable materials to follow a descending path, the first channel being preferably the one which is subjected to subatmospheric pressure. A kind of siphon for the glass to be refined is thus created. This compartment is advantageously equipped with means for adjusting/regulating the head loss of the molten vitrifiable materials at the inlet of the refining compartment. Likewise, the height of the refining compartment may be adjusted depending on various criteria, especially depending on the level of underpressure chosen in the reduced-pressure zone.
According to a second embodiment, the static refining compartment used in the context of the invention is in a vertical orientation and comprises means for introducing the molten vitrifiable materials to be refined in the upper part and means for removing the refined materials in the lower part, the said materials everywhere following a mainly vertical descending path in the said compartment. Its design may, for example, be derived from the teachings from Patents EP-231,518, EP-253,188, EP-257,238 and EP-297,405.
According to a second variant, the refining compartment comprises at least one device capable of being rotated in order to ensure centrifugal refining, the internal walls of the said device substantially defining the shape of a hollow vertical cylinder, at least in its central part. Advantageously, the device comprises a so-called upper zone at subatmospheric pressure and a so-called lower zone left at ambient pressure, these being separated from each other by one or more mechanical means of the type consisting of a metal plate drilled with one or more holes.
According to a preferred design, the device is fed at the upper part with molten vitrifiable materials by a static supply means of the flow-channel type. These supply means may comprise at least one compartment at reduced pressure in order to allow the device to be fed and/or to allow a first refining operation to be carried out.
Sealing means have to be provided in order to join the end of this channel/these supply means to the device, the sealing means being of the xe2x80x9cdynamic sealxe2x80x9d type or rotating seal, as explained in detail below. The device is advantageously provided with means for trapping solid particles having a density greater than that of the glass, these means especially being located in its lower zone and being in the form of notches/grooves made in its internal walls. Preferably, the speed of rotation of the device is selected to be between 100 and 1500 revolutions per minute.
The apparatus may also be provided with mechanical means which are stationary or which follow its rotation, and are capable of shearing the foam and of driving it downwards into the lower zone of the device from which the refined glass is drawn off.
These means are especially in the form of pierced deflectors, or fins placed in the upper zone of the said device.
This type of centrifugal refining, with passage into a reduced-pressure zone, is particularly effective. This is because the reduced pressure will allow the greatest possible increase in the bubbles before the centrifugal refining proper: the bubbles are removed more rapidly in the device the larger their diameter. The reduced pressure will also make it possible to further decrease the residual sulphate content of the glass produced. It should be noted that a desulphated glass (this remark also applies to the first variant in which the refining is carried out statically) gives fewer problems of volatile compounds in the float bath, a reduced risk of the formation of tin sulphide and therefore finally a reduced risk of a tin defect in the sheet of glass. This also guarantees the absence of sulphides in the case of reduced glasses, especially iron sulphides which give not very desirable yellow/orange residual colours or inclusions of nickel sulphide which may cause the glass to break during heat treatment of the quenching type.
The centrifugal refining comprising a reduced-pressure phase is particularly indicated in the case of the refining of relatively foamy glass.